Infections with drug resistant strains of Mycobacterium tuberculosis and most strains of Mycobacterium avium are challenging to treat chemotherapeutically. As HIV infection and the AIDS epidemic spreads, adequate treatment of mycobacterial infections in both immunocompetent and immunocompromised patients becomes increasingly difficult. Additional understanding of innate and immune mechanisms of resistance as well as new antibiotics will be important for designing new, more effective treatment regimens for mycobacterial infection. We hypothesize that small, cationic human antimicrobial peptides (AMP), defensins and cathelicidin stored in granules of human polymorphonuclear leukocytes (PMN), and secreted from epithelia play important roles in resistance to mycobacterial infection, and may be adaptable for chemotherapeutic use. Human neutrophil peptides 1,2, and 3 (HNP 1,2, and 3) have shown to be anti-mycobacterial for laboratory and clinical isolates of M. tuberculosis and M. avium in vitro, and we have confirmed this in our laboratory. We propose to extend these observations by fully characterizing the anti-mycobacterial activity of human AMP, examining the relationship between binding of AMP to mycobacteria and anti-mycobacterial activity, and evaluating the relationship between resistance to human AMP and pathogenicity. We will also examine pharmacological interactions between human AMP and first-line anti-tuberculosis drugs for mycobacteria in liquid and macrophage cultures. The proposed studies are designed to fill an important gap in our knowledge of how mycobacteria evade the innate immune system. Understanding of the relative sensitivities of mycobacteria will help us understand how mycobacteria survive their first encounter with the antimicrobial peptides in pulmonary secretions and in responding neutrophils. Correlation of AMP sensitivity with binding of the peptides will help determine whether and how AMP sensitivity is related to pathogenicity. In addition, the proposed studies will define the potential of AMP to participate in the anti-mycobacterial response of human macrophages, or whether intracellular mycobacteria are protected from exposure to AMP. These studies will further our understanding of how endogenous anti-mycobacterial substances may enhance resistance to infection, and will also reveal whether these substances may be adaptable to chemotherapeutic use for treatment of mycobacterial infections.